Fischer-Tropsch synthesis is carbon monoxide hydrogenation that is usually performed on a product stream from another reaction including but not limited to steam reforming (product stream H2/C0˜3), partial oxidation (product stream H2/C0˜2), autothermal reforming (product stream H2/C0˜2.5), CO2 reforming (H2/C0˜1) coal gassification (product stream H2/C0˜1) and combinations thereof.
Fundamentally, Fischer-Tropsch synthesis has fast surface reaction kinetics. However, the overall reaction rate is severely limited by heat and mass transfer with conventional catalysts or catalyst structures. The limited heat transfer together with the fast surface reaction kinetics may result in hot spots in a catalyst bed. Hot spots favor methanation. In commercial processes, fixed bed reactors with small internal diameters or slurry type and fluidized type reactors with small catalyst particles (>50 μm) are used to mitigate the heat and mass transfer limitations. In addition, Fischer-Tropsch reactors are operated at lower conversions per pass to minimize temperature excursion in the catalyst bed. Because of the necessary operational parameters to avoid methanation, conventional reactors are not improved even with more active Fischer-Tropsch synthesis catalysts. Detailed operation is summarized in Table 1 and FIG. 1.
TABLE 1Comparison of Residence Times Effects in Fischer-Tropsch ExperimentationResidenceCH4Ref(A)CatalystConditionstimeConversionselectivity1Co/ZSM-5240° C., 20-atm, H2/CO = 23.6-sec60%21%2Co/MnO220° C., 21-atm, H2/CO = 20.72-sec 13%15%3Co—Ru/TiO2200° C., 20-atm, H2/CO = 2  3-sec61% 5%Co/TiO2″  8-sec49% 7%4Co/TiO2200° C., 20-atm, H2/CO = 2.1  2-sec9.5%~9%″ 12-sec72%~6%5Ru/Al2O3222° C., 21-atm, H2/CO = 34.5-sec20%?″7.2-sec36%″8.4-sec45%″9.6-sec51%″ 12-sec68%″ 14-sec84%6Ru/Al2O3250° C., 22-atm, H2/CO = 27.2-sec38% 5%7Ru/Al2O3225° C., 21-atm, H2/CO = 2 12-sec66%13%222° C., 21-atm, H2/CO = 3 12-sec77%34%For references that contained results for multiple experimental conditions, the run which best matched our conversion, selectivity and/or conditions was chosen for comparison of residence time.(A) References1. Bessell, S., Appl. Catal. A: Gen. 96, 253 (1993).2. Hutchings, G. J., Topics Catal. 2, 163 (1995).3. Iglesia, E., S. L. Soled and R. A. Fiato (Exxon Res. and Eng. Co.), U.S. Pat. No. 4,738,948, Apr. 19, 1988.4. Iglesia, E., S. C. Reyes, R. J. Madon and S. L. Soled, Adv. Catal. 39, 221 (1993).5. Karn, F. S., J. F. Shultz and R. B. Anderson, Ind. Eng. Chem. Prod. Res. Dev. 4(4), 265 (1965).6. King, F., E. Shutt and A. I. Thomson, Platinum Metals Rev. 29(44), 146 (1985).7. Shultz, J. F., F. S. Karn and R. B. Anderson, Rep. Invest. - U.S. Bur. Mines 6974, 20 (1967).
Literature data (Table 1 and FIG. 1) were obtained at lower H2/CO ratio (2:1) and longer residence time (3 sec or longer). Low H2/CO (especially 2-2.5), long residence time, low temperature, and higher pressure favor Fischer-Tropsch synthesis. Selectivity to CH4 can be significantly increased by increasing H2/CO ratio from 2 to 3. Increasing residence time also has a dramatic favorable effect on the catalyst performance. Although reference 3 in Table 1 shows satisfactory results, the experiment was conducted under the conditions where Fischer-Tropsch synthesis is favored (at least 3 sec residence time, and H2/CO=2). In addition, the experiment of reference 3 was done using a powdered catalyst on an experimental scale that would be impractical commercially because of the pressure drop penalty imposed by powdered catalyst. Operating at higher temperature will enhance the conversion, however at the much higher expense of selectivity to CH4. It is also noteworthy that residence time in commercial Fischer-Tropsch units is at least 10 sec.
Hence, there is a need for a catalyst structure and method of Fischer-Tropsch synthesis that can achieve the same or higher conversion at shorter residence time, and/or at higher H2/CO.